Most metropolitan areas are now equipped with one or more forms of wireless communication networks which provide mobile telephone and other related services to customers across a broad frequency spectrum. Consider, for example, what has come to be known as "cellular" telephone services or Personal Communication Services "PCS", i.e., radio transmissions in the frequency band between approximately 800 MHz and 2.2 GHz.
Public wireless communications services are removing the historical mobility constraints on the users of public communications systems. Continuing improvements in the capabilities and services offered by these systems will be accompanied by increasingly broad public acceptance and adoption. Relaxing the mobility constraints on communications systems users is thus generally considered desirable. For service providers who rely on precise location information, however, such as for example, Emergency "911" (E-911) service providers, this increased user mobility is a definite disadvantage. Uncertainty concerning the user's location translates directly into increased response times and therefore a reduced quality of service when the caller is uncertain of, or cannot describe, her location.
In wired commercial communications systems, precise information concerning the location of the user's equipment, and hence that of the user, is available. Wireless systems, on the other hand, typically follow a cellular model in which there are wired connections to base stations and then wireless links from each base station to its active users. The service area for each base station is roughly circular in shape with a radius of 5-20 km and overlaps that of its neighboring base stations. This corresponds to a base station service area in the range of roughly 80-1300 km.sup.2. In some systems, the coverage area for each physical base station is further divided into equiangular sectors, each operating as an independent cell. Three 120 degree sectors is representative of this type of design, and the corresponding sector service areas become 26-420 km.sup.2. Although the serving base station (and sector) of a wireless E-911 caller, for example, are known, the corresponding location information is clearly too coarse for timely E-911 response. FIG. 1 of the drawings depicts this described situation for conventional omnidirectional or sector base stations.
As shown in FIG. 2, prior art cellular telephone systems 10 include a mobile telephone switching center (MSC) 12 and a plurality of base stations such as cell site transceivers 14a-14c. The cell site transceivers transmit radio signals to and receive radio signals from one or more mobile units 16 that move about a cellular service area 20. A mobile unit, as the term is used herein, refers to a wireless voice telephone or data receiver that can be permanently installed at a fixed location or within a vehicle or that can be portable. Each cell site transceiver 14 is able to broadcast and receive the radio signals within a geographic area 18 called the cell site coverage area. Together, the areas 18 comprise the entire cellular service area 20. Typically, a cellular service area comprises a metropolitan area or larger region.
When a telephone call to a called mobile unit 16 originates from either another mobile unit or a land-based telephone via a Public Switched Telephone Network (PSTN) 22, a caller must first access the cellular telephone system 10. This task is accomplished by dialing the mobile unit's unique identification number (i.e., its phone number). The MSC 12 receives the call request and instructs the central call processor 24 to begin call processing. The central call processor 24 transmits a signal over a dedicated line 26 (such as a telephone line or microwave link, etc. (to each of the cell site transceivers 14a-14c causing the cell site transceivers to transmit a page signal that the mobile unit 16 receives. The page signal alerts a particular mobile unit 16 that it is being called by including as part of the page signal the paged mobile unit's identification or phone number.
Each cell site transceiver 14 transmits the page signal on one or more dedicated forward control channels that carry all pages, as well as control signals, channel assignments, and other overhead messages to each mobile unit. The forward control channel is distinct from the voice channel but actually carries voice communications between a mobile and another mobile unit or a land-based telephone. Each cell site transceiver may have more than one forward control channel upon which pages can be carried.
When a mobile unit is not engaged in a telephone call, it operates in an idle state. In the idle state, the mobile unit will tune to the strongest available forward control channel and monitor the channel for a page signal or other messages directed to it. Upon determining that a page signal is being transmitted, the mobile unit 16 again scans all forward control channels so as to select the cell site transceiver transmitting the strongest Signal. The mobile unit then transmits an acknowledgement signal to the cell site transceiver over a reverse control channel associated with the strongest forward control channel. This acknowledgement signal serves to indicate to the MSC 12 which of the forward control channels (associated with the several cell site transceivers 14a-14c) to use for further call processing communications with mobile unit 16. This further communication typically includes a message sent to the mobile unit instructing it to tune to a particular voice channel for completion of call processing and for connection with the calling party.
The details of how the cell site transceivers transmit the signals on the forward and reverse control channels are typically governed by standard protocols such as the EIA/TIA-553 specification and the air interface standards for Narrowband Analog Mobile Phone Service (NAMPS) IF-88 and IS-95 air interface standards for digital communications, all of which are well known to those of ordinary skill in the wireless telephone communications art and therefore will not be discussed.
While cellular networks have been found to be of great value to mobile users whose travels span many miles, they have also been found to be prohibitively expensive to implement for small scale applications wherein system subscribers only desire wireless telephone services in limited geographic areas, such as, for example, within office buildings or in campus environments.
The Personal Communications Network (PCN) is a relatively new concept in mobile communications developed specifically to serve the aforementioned applications. Similar to cellular telephony goals, a Personal Communications Network goal is to have a wireless communication system which relates telephone numbers to persons rather than fixed locations. Unlike cellular telephones, however, the PCN telephones are directed to small geographic areas thus defining "microcellular" areas designed to operate in similar fashion to large scale cellular telephone networks. PCN technologies are also similar to residential cordless telephones in that they utilize base stations and wireless handsets. Unlike the former, however, PCN technology utilizes advanced digital communications architecture, such as, for example, PACS, formerly called WACS, (Bellcore), DECT (European), CDMA (Omnipoint), PHS-PHP (Japan), IS-54 (TDMA), IS-95 (CDMA), PCS-1900 (GSM), and B-CDMA (Oki), and features which may be implemented either as private networks or regulated services. When offered by communications carriers as services, this PCN capability is generally referred to as Personal Communications Services (PCS), and may be situated in a wide variety of environments, including, for example, outdoor urban, suburban, rural, indoor single-level and indoor multi-level areas.
As shown in FIG. 3, prior art PCS systems 28 include one or more control units 30 which, in accordance with the American National Standards Institute (ANSI) T1P1 working document for stage 2 service description, as known to those skilled in the art, are termed Radio Access System Controllers (RASCs), access managers, etc. These control units 30 operate in similar fashion to the MTSC 12 of the cellular telephone network and, therefore, are provided in electrical communication with the Public Switched Telephone Network 22. A plurality of base stations or Radio Ports (RPs) 32 are also provided which transmit radio signals to and receive radio signals from one or more subscriber wireless telephones 16, termed mobile units or Radio Personal Terminals (RPTs) that move about a PCS service area 34. Each Radio Port 32, like cell site transceivers 14, is able to broadcast and receive radio signals within a geographic area 36 called the Radio Port coverage area. Together, the areas 36 comprise the entire PCS service area 34.
A generalized reference architecture for the PCS system of FIG. 3 is shown in further detail in FIGS. 4a-4b. The reference architecture includes reference elements which support radio access, wireline access, switching and control, mobility management, and Operations, Administration, Maintenance and Purchasing (OAM&P). As shown in the schematic, the PCS system includes a PCS Switching Center (PSC) 38 which supports access independent call/service control and connection control (switching) functions and is responsible for interconnection of access and network systems to support end-to-end services. The PCS switching center 38 represents a collection of one or more network elements. The system further includes a Radio Access System Controller (RASC) 40 which supports the wireless mobility management and wireless access call control functions. It serves one or more subtending Radio Port Controllers (RPCs) 42 and may be associated with one or more PCS switching centers 38. As known to those skilled in the art, Radio Port Controllers 42 provide an interface between one or more subtending Radio Port Intermediaries (RPIs), a PCS switching center such as 38, and RASC, air interface independent radio frequency transmission and reception functions.
The system further includes a Radio Port Intermediary (RPI) 44 which provides an interface between one or more subtending Radio Ports 46 and the Radio Port Controller 42, and supports air interface dependent radio frequency transmission and reception functions. Radio Port 46 supports the transmission of signals over the air interface and is provided in communication with Radio Personal Terminal (RPT) 48. This is a light-weight, pocket-size portable radio terminal providing the capability for the user to be either stationary or in motion while accessing and using telecommunication services.
The system further includes variations of RPTs which are in fixed locations, termed Radio Termination Type 1 (50) and Radio Termination Type 2 (52), which interface Terminal Equipment Type 1 (54) and Terminal Equipment Type 2 (56) to the Radio Access Interface.
The system of FIG. 4 further includes a Terminal Mobility Controller 58 which provides the control logic for terminal authentication, location management, alerting, and routing to RPT/RTs. There is also provided a Terminal Mobility Data-store (TMD) 60 which is operative to maintain data associated with terminals.
Still further, the system includes a Personal Mobility Controller (PMC) which provides the control logic for user authentication, service request validation, location management, alerting, user access to service profile, privacy, access registration, and call management. PMC 62 is provided in communication with a Personal Mobility Data-store (PMD) which maintains data associated with users.
Finally, the system includes Operations, Administration, Maintenance, and Provisioning, (OAM & P) systems 66 which monitor, test, administer, and manage traffic and billing information for personal communications services and systems. PCS 38 is also provided in communication with Auxiliary Services 68, Interworking Functions (IWF) 70 and External Networks 72. In accordance with the above-referenced working document for Stage 2 service description, auxiliary services 68 are defined as a variety of services such as voice mail, paging, etc. which may not be provided by the PCS 38. IWF 70 are further defined as mechanisms which mask the differences in physical, link and network technologies into consistent network and .user services. Still further, external networks 72 are defined as other voice, digital data, packet data, and broadband data networks.
FIG. 5 provides a unified functional model of the detailed system of FIGS. 4a-4b. This functional model is derived from the PCS reference architecture in FIG. 4 by aggregating the terminal entities (RT and RPT) into a single functional grouping RTF, and aggregating RP, RPI, and RPC into another single functional grouping RCF in accordance with the Stage 2 service descriptions for PCS. The model includes Call Control Function (CCF) 74, Service Switching Function (SSF) 76, Service Control Function (SCF) 78, Service Data Function (SDF) 80, Service Resource Function (SRF) 82, Radio Control Function Access Control Function (RACF) 84, Radio Control Function (RCF) 86, and Radio Termination Function (RTF) 88. The functions of the terminal elements are more fully described in the Stage 2 service description for PCS.
Wireless communication services such as the above cellular and PCS systems, have been quickly embraced by those people whose business requires them to travel frequently and to be in constant communication with their clients and associates. The increased use of wireless communication services, however, have caused headaches for emergency operators and other position dependent service providers who require precise location data. As known to those skilled in the art, under current wireless technology, position data is strictly limited to relatively large coverage areas and sectors thereof as defined by the RF characteristics, i.e. footprints, of the associated base station. As explained below, these coverage areas are generally unsuitable for most commercial and consumer applications.
In the late 1960's, federal legislation was enacted which established the 9-1-1 telephone number as a national emergency resource. In land-based systems, Enhanced 9-1-1 (E 9-1-1) wireline technology provides the caller's Automatic Location Identification (ALI) with reasonable accuracy, cost and reliability, to a Public Safety Answering Point (PSAP) via a defacto standard. ALI is generally accomplished by receiving the ANI, or Automatic Number Identification, during call setup to the PSAP. A database query, given ANI, provides ALI to the emergency call taker display terminal as both parties establish the voice channel.
Currently, however, wireless technology does not provide ALI. As a result, an ever-increasing percentage of emergency telephone calls can be tracked no further than the originating base station. As readily seen, the heart of the problem for providing E9-1-1 ALI services for wireless communication customers lies in accurately and reliably determining the handset location, under any circumstance, at low cost.
Against this background, there have been previous attempts to provide methods and systems which generally identify the positions of wireless communication system users in cell site coverage areas and sectors thereof. See, for example, U.S. Pat. No. 4,876,738 issued to Selby and assigned to U.S. Phillips Corporation. Selby discloses a registration procedure in which the base station monitors the location of the mobile unit by cell site. The effect is to allow enlargement of the registration area if the mobile unit consistently roams between two cells.
See also, U.S. Pat. No. 5,179,721 issued to Comroe et al and assigned to Motorola, Inc. Comroe discloses a method for inter-operation of a cellular communication system and trunking communication system by transmitting an access number for each system such that the mobile unit may be used as a cellular telephone and a trunking communication device.
Still further, see U.S. Pat. No. 5,097,499 issued to Consentino and assigned to AT&T Bell Laboratories. Consentino teaches a method for preventing an overload in a reverse channel by delaying the time of the generation of timing stamps on markers.
These methods and systems, however, have proven unsuitable for commercial and consumer applications where users may, at any given time, travel through very small portions of numerous cell site coverage areas and sectors. Under current wireless technology, and as described in the prior art referenced above, presently available positioning methods and systems are limited to a determination of whether the user is within one or more predetermined cell site coverage areas or sectors. These prior art systems are incapable of providing further detail, i.e. exactly where in the cell site coverage area the user is located.
Prior art attempts to design higher accuracy positioning systems which utilize commercial broadcast transmissions, for example, have also met with limited success. See, for example, U.S. Pat. Nos. 4,054,880 (Dalabakis et al) and 3,889,264 (Fletcher) which disclose what are known as "delta-position" systems. These prior art patents describe systems using three spectrally spaced-apart radio signals, each of which is an independent AM radio signal. The systems typically have a vehicle carried mobile receiver, with a separate tuner for each station, and a second receiver at a fixed, known position. As disclosed, these systems count "zero crossing counts", each of which indicates that the user has moved a certain distance from his or her previous location. In operation, if it is desired to determine the current position of the user, a starting position must first be specified. A fixed position receiver detects frequency drift of the transmitters, which is used to adjust and coordinate zero crossing counts made by the mobile receivers.
These systems are termed "delta-position" systems because they determine only the distance and direction traveled by a mobile user from any particular starting point. Neither Dalabakis et al nor Fletcher actually determines the position of the mobile user.
See also, U.S. Pat. No. 5,173,710 to Kelley et al which discloses the use of a fixed position receiver which is adapted to determine frequency drift along with the relative phases of various unsynchronized FM broadcast signals originating from known fixed locations. As disclosed by Kelley, each of the fixed transmitters transmits a beacon signal having a phase that is unsynchronized with the phases of the beacon signals of the other transmitters. These signals are 19 kHz analog pilot tones generated by commercial broadcast stereo FM stations. The fixed receiver receives the beacon signals, determines the relative phases of the beacon signals, and broadcasts data representing these relative phases for receipt by the mobile receiver which is at an unknown location. Each mobile receiver includes phase measurement circuitry that detects the phases of the beacon signals at the mobile receiver's current position on multiple distinct carrier frequencies such that the current position of the mobile unit may be determined when used in conjunction with the fixed receiver broadcast data.
See also, U.S. Pat. Nos. 5,055,851; 4,891,650; and 5,218,367, all issued to E. Sheffer and assigned to Track Mobile, Inc. Like the '650 patent, the '851 patent utilizes measurements of the mobile unit's signal strength which is detected by some number of neighboring base stations in order to calculate location. In operation, each base station transmits a special packet of data which includes this information for receipt by the MTSC. Another packet of information, the actual vehicle alarm distress call (this is not the same as a 9-1-1 call), is also sent to the MTSC. The MTSC sends these two information packets to a Track Mobile alarm center personal computer. The computer matches both packets using a simple algorithm in order to find the vehicle's distance from the base station cell center point. As disclosed, this is done preferably with four neighboring base station cell site measurements along with arcuation or line intersection techniques. The results are displayed on a computer screen map. A 9-1-1 call may then be initiated by a Track Mobile attendant, based on a verbal request from the originating mobile user.
The '367 patent operates in much the same way although it uses a modified handset including a modem, to send signal strength measurements received at the mobile unit, through the cellular network to the Track-Mobile alarm center. Only the downlink signal strengths, received at the mobile unit, are used to estimate location. The location is determined from the same algorithm as in the '851 patent, but includes a refinement--antenna sector ID--if known. As disclosed, the sector ID information reduces error by effectively slicing the cell circle into one of three pie-shaped sections. In the case of low power PCS installations, it is likely that omnidirectional antennas would be used, thus precluding the use of this sector refinement.
None of the systems referenced above, as well as general time difference of arrival location systems such as LORAN, NAVSTAR, and GPS, as used for example in U.S. Pat. No. 4,833,480, issued to Palmer et al, have proven suitable for commercial applications since, by design, they require specially adapted receivers to receive and process the pilot tones, GPS signals, etc. at the mobile unit. This sophisticated end equipment, of course, significantly adds to the cost of the corresponding mobile unit. In the case of hand portable units, this additional equipment further results in a handset which is extremely bulky and difficult to handle. As a result, these systems have proven unsuitable for both large scale commercial applications, as well as ordinary consumer use.
When applied to wireless communications of interest to the present invention, i.e. communications in the frequency band from 800 MHz to 2.5 GHz, these prior art systems are further considered unsuitable for commercial applications in view of their anticipated use of excessive frequency spectrum. More specifically, it is anticipated that for proper operation, these systems would necessarily require transmission of signals on separate channels which would utilize an unacceptable amount of additional spectrum.
Still further, the prior art systems fail to account for changes in environmental conditions. For example, it is known to those of skill in the art that for GPS receivers, the location calculation will not work unless there is a clear view of at least 3-4 satellites. In dense urban areas, especially at the street level, this condition could easily prevail. Thus, no location estimate would be available if less than three satellite signals can be received.
In many office buildings, the metal content of the windows is also sufficient to preclude effective satellite reception. To this end, if all wireless antennas were isotropic and were located in flat and open terrain, estimating the location of a handset/mobile unit using the prior art Trackmobile signal strength technology might be sufficient. Unfortunately, the known advantage of the PCS world, and to a reasonable extent, cellular, does not have a flat and open terrain. None of the prior art patents work in areas where there are obstructions to the radio signal's path like buildings, trees, hills, and automobiles.
Seasons are also known to have a dramatic affect on propagation where radio waves are significantly attenuated by tree leaves in the summer, but less so in the winter. Thus, actual RF field data gathered in one season may not be accurate in another season. As readily seen, precisely predicting location based on RF propagation loss has generally been an intractable problem, due to the complexity of factors, as well as the data collection difficulties in constructing the necessary databases needed to supply the actual field data. Thus, the principles relied upon by the above-referenced prior arts patents, free space loss, rarely exists, as obstructions and interference increases daily, even in the most optimal RF environments.
Consequently, a need has developed to provide a positioning method which may be practically and economically implemented for use in wireless communication systems and, in particular, in the microwave band from 800 MHz to 2.5 GHz.
Still further, a need has developed to provide such a method which may be used by service providers to provide precise location information for use in emergency situations such as locating an Enhanced 9-1-1 (E9-11) 1) caller, enforcing restraining orders and house arrests, confirming the intended location of a user at predetermined times and the like. It is further desirable that such a method be compatible with existing wireless telephone technology and should not degrade the operation of an existing system. Finally, such a system should neither require the allocation of more radio frequencies than are currently allocated to wireless telephone systems, nor require a substantial portion of existing wireless frequencies.